Air purifiers are commonplace in today's society to clean air in confined spaces, e.g. rooms, for example to reduce the exposure of people in such confined spaces to harmful or unpleasant pollutants, e.g. allergens, particles, odours, and so on. To this end, the air purifiers typically comprise one or more pollutant removal structures, such as one or more filters, catalytic converters, electrostatic precipitators, and so on. The one or more filters may include air filters such as carbon filters, HEPA filters, odour filters, anti-bacterial filters or the like. Catalytic converters may be used to break down gaseous pollutants into smaller molecules, e.g. H2O and CO2. Electrostatic precipitators may be employed for the removal of charged particles via collector plates. Other pollutant removal technologies employed in such purifiers are also known.
In order to control the operation of the air purifier, the air purifier may contain pollutant sensors, e.g. to regulate the air flow through the air purifier based on measured pollutant concentrations in the inbound air or to monitor the performance of the pollutant removal structures, which typically have a limited lifespan and must therefore be regularly replaced or serviced in order to ensure that the air purifier exhibits the desired performance characteristics, i.e. sufficiently purifies the air in a confined space in which the air purifier is placed, e.g. a room of a building such as an office space or a house.
A commonly used figure of merit for air purifiers is the clean air delivery rate (CADR), which expresses the fraction of pollutants that have been removed from the air times the CFM (cubic feet per minute) air flow rate through the air purifier.
However, it is not straightforward to predict when such pollutant removal structures need replacing or maintenance. Given that such pollutant removal structures, e.g. air filters, can be rather costly, it is desirable that such pollutant removal structures are not prematurely replaced or serviced as this can significantly increase the operating cost of an air purifier. On the other hand, if pollutant removal structure replacement or servicing is delayed beyond its end of life (EOL), the performance of the air purifier including the pollutant removal structure may become insufficient, which may lead to health problems for people occupying the confined space in which the air purifier is positioned. This is particularly prevalent for certain risk groups; it is well documented that pregnant women, infants/children, elderly and people with respiratory or cardiovascular disease are at increased risk from pollution exposure. For these groups, there is an enhanced need to minimize their exposure to air pollution.
Several air purifier manufacturers maintain fixed EOL values for the pollutant removal structures in the air purifier such that a user is prompted at regular intervals to replace or service such structures. However, this approximation does not factor in environmental conditions and operating times and may therefore lead to rather inaccurate approximations of the EOL of these pollutant removal structures.
According to GB/T 18801 standard, the lifetime of an air filter is reached once its initial CADR falls to 50%. The decrease in CADR, which for instance is caused by ageing of the air filter, correlates with an increase in the time required to reduce the room concentration of a pollutant. One of the attractions of such a standard is that it enables consumers to compare the standardized performance of different air purifiers such that an air purifier best matching a consumer's need can be more easily identified.
However, this standard is still based on generalized assumptions, e.g. 12 hours per day running time, fixed air exchange rates etc., without taking into account the actual usage of the air purifier. Since the actual life time depends on multiple factors related to the actual usage as introduced above, the problem arises that most of the users, when following the manufacturer's recommendations, will replace or service their pollutant removal structure either too early, which causes unnecessary costs or too late, which means that the pollutant removal structure is still used even when it is no longer able to reduce a pollutant level to a safe value.
CN 102019102 A discloses a method for real-time monitoring of the pollution level of a filter layer of an air purification machine, comprising a detection method of air quality. The method is characterized in that air quality sensors are arranged at the air inlet end and the air outlet end of the filter layer and used for detecting the air quality at the two sides of the filter layer; and an operation circuit judges the pollution level of the filter layer according to air quality signals detected by the two air quality sensors. When the filter layer is severely polluted so that the purification effect is poor, an alarm signal can be sent out to remind a user of timely changing or washing the filter layer, thereby ensuring the use effect of the air purification machine. The need to integrate multiple air quality sensors into the air purification machine increases its cost, which may be undesirable. Moreover, the use of multiple air quality sensors may complicate the accurate determination of the filter layer EOL, for example because the respective air quality sensors exhibit a different drift in sensitivity over time, e.g. because the air quality sensor in front of the filter layer becomes more contaminated with pollutants than the air quality sensor behind the filter layer.
US 2003/181158 A1 discloses an economizer control, which includes a sensor that senses characteristics of air, a damper located relative to the sensor so that the damper can control air flow of outside air and re-circulated air to the sensor, and a controller in communication with the sensor and the damper. The controller controls the opening and closing of the damper according to conditions sensed by the sensor.
US 2012/145010 A1 discloses an air cleaner, a case of an air cleaner includes an inlet, through which the outside air is taken in by rotation of an internal fan. The outside air taken in passes through a filter from the inlet in a first flow path. A sensor unit is arranged outside of the first flow path, and the outside air taken in passes through the sensor unit from the inlet by a flow path pipe forming a second flow path different from the first flow path. As a result, the outside air taken in through the flow path different from the flow path toward the filter is sensed by the sensor unit.